This invention relates to optical data storage. In particular, a focussed light beam is used to read or write data on a recording layer which is one of several layers of various materials which form a media. The invention includes the construction of the media, placement of the recording layer within the media, the optical path within the media, construction and placement of a head to read and write the media, and the construction of a double-sided media.
In a typical, known type of optical data storage system, either write-once or rewritable, a media is formed with a layer of recording material covered by a transparent substrate material. In a read-only media, the data would be placed in the recording layer during a manufacturing process, for example by a masking or embossing process. In a write-once or rewritable media, data is written into the recording layer during a writing process, which usually includes increased power and intensity of a laser light beam to alter the optical properties of the recording layer in a localized region. A lower intensity light beam can be used to read data from the recording layer.
The media can be formed as a disk which rotates about its center, and an optical head emitting a focussed light beam moves to various radial positions above the disk to access different portions of the disk. The light beam is focussed through an objective lens to enter through a top surface of the media nearest the optical head, travel down through the substrate material, and come to a point of focus on the recording layer located substantially at the bottom surface of the media. Dust or other contamination on the top surface will be out of focus and thus not interfere with the reading or writing process on the recording layer. Dust on the bottom surface does not matter because it is beyond the recording layer on which the optical beam focuses. For a given numerical aperture of the objective lens, the distance L from the entry surface to the recording layer must be greater than some minimum length to give an adequate dust defocusing effect. A double-sided media would place two such media back-to-back and have a thickness of at least 2 L or twice the thickness of a single media, making a double-sided media incompatible with mechanisms set for handling the thickness of a single media.
In a particular type of rewritable optical data storage known as magneto-optical storage, a strong magnetic field is required in the region of the focussed light beam when writing data. Several schemes require modulation of the magnetic field at frequencies comparable to the rate of writing the data, and thus the size and inductance of the magnetic head generating the magnetic field are kept small to achieve fast rates and to keep power consumption low. The requirement of small inductance forces use of a physically small magnetic head which must then co-move with the optical head as it accesses different regions of the media to bring sufficient magnetic field to the area being accessed. The magnetic head should also be positioned as close as possible to the recording layer within the media so that adequate field strength is achieved.
In typical, known magneto-optical systems, the optical head is placed above the top surface of the media and the light beam focuses down through the substrate to the recording layer, while the magnetic head is placed below the bottom surface of the media which brings it closer to the recording layer on the bottom surface. This requires movement and positioning mechanisms on both sides of the media which must move together across the disk. This greater complexity leads to increased size, cost and power consumption, slower access times, and less reliability. Because the media is simultaneously accessed from two sides, there is no way to increase storage capacity by using double-sided media. If the magnetic head is moved to the top surface and combined with the optical head, in order to simplify the positioning mechanisms, the magnetic head may be too far from the recording layer for adequate field strength.
The invention of this application allows an optical media to be half as thick for the same full dust defocusing distance L. This allows a double-sided media to be constructed in the same thickness as a standard single-sided media. Alternatively, in a standard thickness media, greater dust defocusing effect can be obtained. When used in a magneto-optical system, this invention allows the optical and magnetic head to be combined on the same side of the media, while bringing the magnetic head to be very close to the recording layer. The combined optical and magnetic head is less costly and more reliable than dual or separate heads. Since access to the media can be accomplished from one side, a double sided media can have two independent or simultaneously operating heads, one on each side, to reduce access times or increase transfer rates. These advantages are available to read-only, write-once, and rewritable optical systems in disk, tape, or card formats, in single or double-sided forms.